Pressurized water nuclear reactors comprise steam generators which ensure the heating and vaporization of feed-water via the heat conveyed by the pressurized water used to cool the core of the reactor. Pressurized water reactors, on each of their primary branches, comprise a steam generator having a primary part in which the pressurized water for cooling the reactor circulates, and a secondary part receiving the feed-water which is heated and progressively vaporized and then leaves the secondary part of the steam generator in the form of steam which is sent to a turbine associated with the nuclear reactor to ensure the driving of an alternator producing electric current.
Said steam generators comprise an outer shell called a pressure shell generally of cylindrical shape and arranged with its axis positioned vertically and secured to a substantially horizontal tube sheet whose lower face or inlet face forms a wall of the plenum of the steam generator fed with pressurized water forming the primary fluid.
The steam generator also comprises a bundle of U-bend tubes, each comprising two straight branches parallel to one another whose ends are secured in holes passing through the tube sheet between the lower inlet side of the tube sheet and the upper outlet side from which the tubes of the bundle are held in position in the secondary part of the steam generator, in which part an inner wrapper, called the bundle wrapper arranged in coaxial position inside the pressure shell, delimits an annular space with this pressure shell.
The tubes of the bundle are additionally held in regular position over transverse planes perpendicular to the axis of the bundle, via tube support plates which are distributed at regular distances over the height of the bundle.
A network of openings passes through the tube support plates, this network being identical to the network of openings of the tube sheet of the steam generator.
These tube support plates which hold the tubes of the tube bundle within the bundle wrapper are provided with flow holes for secondary water along the tubes of the bundle, the holes passing through the thickness of each tube support plate and being lobe-shaped, called trefoil or quatrefoil flow holes.
During the functioning of the steam generator, the circulating feed-water in contact with the outer surface of the tubes of the bundle and inside the secondary circuit of the reactor, picks up impurities of various types and in particular of magnetite type which may be deposited in the form of sludge in different parts of the steam generator and in particular in the spaces between the tubes of the bundle and the flow holes provided in the tube support plates to allow the coolant water to circulate around the tubes of the bundle.
The deposits which accumulate between the flow holes of the support plates and the outer surface of the tubes of the bundle progressively clog these flow holes preventing circulation of coolant water and may produce some encasing of the tubes in the holes, which means that the tubes become rigidly fixed in the tube support plate preventing their movement in the axial direction common to the flow hole and the tube, and also causing their immobilization in the radial directions.
Under these conditions, the thermal yield of the steam generator is largely degraded, and in extreme cases there is a risk that the tubes may be subjected to stress above the tube support plates and may subsequently become cracked, causing a leak of primary water into the secondary circuit leading to shutdown of the nuclear reactor.
To measure the fouling rate of these deposits which accumulate between the flow holes of the tube support plates and the outer surface of the tubes, a probe is known from FR-2 927 723 also to the Applicant, this probe comprising a body of general cylindrical shape having on its outer surface at least two magnetic field detectors arranged co-axially and secured to said body, and containing at least one permanent magnet co-axial to this body whose North-South orientation coincides with the longitudinal axis of the body, said at least two detectors being connected by connection means to a system amplifying and displaying variations in the signals transmitted by these detectors when the probe is moved inside a tube of the steam generator. FR 2928024, FR 2459490 and DE 4338752 also teach probes for detecting deposits. U.S. Pat. No. 5,256,966 discloses a method and apparatus for inspecting the profile of the inner wall of a conduit by a probe body that includes eddy current sensing coils. FR 2692044 discloses a robotic sampling device for cutting a part of a tube wall for sampling where the sample can be mounted onto a separate tube for testing the physical properties of the cut wall portion and a video probe can be passed through the window to monitor conditions near the support plates and tube sheets.
Each flow hole of the tube support plates is shaped with three of four lobes or alveoli, called trefoil or quatrefoil respectively.
Therefore, each flow hole is formed of alternating alveoli and solid portions in contact with a tube.
Measurement of fouling rate using this type of probe corresponds to a mean for the flow hole as a whole i.e. the total number of alveoli.
Yet, this phenomenon of fouling in each of the flow holes is not uniform and one alveolus may be clogged to a greater extent than neighbouring alveoli. In addition, the localization of the deposits in a given alveolus is variable and they may lie preferably on the tube side or on the side of the tube support plate, or they may be uniformly distributed around the entire periphery of the cross-section of the flow hole. On account of its operating principle, the above-mentioned probe is sensitive to the distance between the measured deposit and the detector and therefore entails some measurement uncertainty related to the geometry of the deposit.